Particulate and gaseous exhaust emissions were measured on a sample of six diesel engined road vehicles of less than 3.5 tonnes gross vehicle weight. Measurements were made in the laboratory and on the road over a wide range of operational conditions including ECE 15 and US Federal test drives, steady state drives at several speeds and rural, urban and motorway drives over prescribed routes. All of the vehicles tested had gaseous emissions that were within the ECE Regulation 15-04 type approval limits: CO emissions were less than 10 per cent and combined HC and NOx emissions generally around 30 per cent of the limit values. It was estimated that the pre and post 1987 US particulate standards (0.6 and 0.2 g/mile) would be equivalent to about 0.44 g/km (1.78 g/test) and 0.15 g/km (0.61 g/test) respectively over the ECE 15 test. Only one vehicle (a directly injected vehicle) emitted more than 0.44 g/km whereas only one vehicle emitted less than 0.15 g/km. The variation of emission rates with vehicle speed was examined and it was shown that the average CO emissions from all six vehicles fell as the road speed increased, with some evidence from two vehicles of an upturn at the highest speed. NOx emissions in general also became lower as the speed increased. The general trend of particulate emissions was to fall with increasing speed. Some cars showed an increase at higher speeds. Visible smoke was also measured during the dynamometer tests but no relationship was observed between smoke and the rate of mass emission of particulates. (A)